This invention relates to a process for the production of solid polyisocyanates stabilized by a polymer coating and having retarded reactivity. The stabilized polyisocyanates are prepared by reacting solid, finely particulate polyisocyanates with di- and/or higher functional compounds containing aliphatically bound amino groups and/or terminal --CO.NH--NH.sub.2 -groups and/or hydrazine(s) in a quantity of from 0.1 to 25 equivalent percent of "amine" per equivalent of NCO in a liquid medium of monoalcohols and/or polyols and/or polyamines and/or plasticizers and/or water (optionally in the presence of apolar or slightly polar solvents) to form a suspension of polyadduct-coated, stabilized polyisocyanates in the liquid medium. The resultant polyisocyanates may be isolated, optionally by filtration and then suspended in polyols and/or polyamines.
The invention also relates to polyadduct-coated polyisocyanates obtained by the above process in finely particulate form, preferably suspended in low molecular weight and/or relatively high molecular weight polyols and/or in low molecular weight and/or relatively high molecular weight aromatic and/or relatively high molecular weight aliphatic polyamines, characterized by a 0.1 to 25% conversion of the NCO-groups and by a higher "thickening temperature" than the corresponding unstabilized polyisocyanates.
The present invention also relates to the use of the stabilized polyadduct-coated polyisocyanates optionally suspended in polyols and/or polyamines as reaction components in polyurethane syntheses, preferably using aromatic polyamines as chain-extending agents. More particularly, the compositions of the present invention may be used in hardenable reactive polyurethane systems having a long pot life.
The one-component reactive mixture can be converted into polyurethanes, preferably by thermal hardening, optionally using lead and/or tin catalysts.
Storable one-component systems based on polyisocyanates which are solid at room temperature and which show high stability in storage, even with respect to aromatic polyamines, have never been described before. There are also very few publications on the surface modification of polyisocyanates solid at room temperature.
German Offenlegungsschrift No. 25 57 407 describes a process in which a solution of a polyisocyanate in a low-boiling solvent is sprayed into a reactor with gaseous di- and/or polyamine, the reaction occurring between the polyisocyanate and the amine. The reaction is followed by evaporation of the solvent, yielding hollow polyurethane polyurea beads which are preferably used as fillers. The reaction is carried out in such a way that virtually all the NCO-groups react with the amine and with any other NCO-reactive components added. There is no indication of the reaction being carried out in such a way that the polyurea coating makes up only a fraction of the solid. There is also no indication that unreacted NCO-groups in the interior are available for further reactions in the sense of a one-component polyurethane system.
U.S. Pat. No. 3,409,461 describes the coating of polyisocyanates with a protective substance, preferably a polymer, as a result of which the polyisocyanate particles are surface-deactivated. To accomplish this, the isocyanate is dispersed in a solution of the polymer in a low-boiling solvent which does not dissolve the isocyanate to any significant extent. The dispersion thus formed is spray-dried. In a preferred embodiment, finely ground (particle size of from 1 to 10 .mu.m) naphthylene-1,5-diisocyanate is spray-dried with a 1 to 2.5% solution of polystyrene, polyvinyl butylether, chlorinated rubber and the like in tetrachloromethane. Free-flowing powders having a particle size of from about 1 to 50 .mu.m are obtained. They are preferably used for improving the adhesion of polyester products (fabrics, fibers, films) to rubber elastomers. In this process, it is necessary to use considerable quantities of solvents (which may be toxic), for example 4 kg of tetrachloromethane for 50 g of naphthylene-1,5-diisocyanate, which then must be removed by an energy-consuming process. One particular disadvantage of the process is the considerable contribution of the coating to the total weight of the coated isocyanate, amounting to between 9 and 91% by weight and generally to around 50 % by weight. An excessive percentage of troublesome foreign substance would, thus, be introduced in the production of high-quality polyurethanes.
U.S. Pat. No. 3,551,346 describes the encapsulation of liquid diisocyanates by interfacial reactions of CH.sub.3 --Si--(OCH.sub.3).sub.3 dissolved in the diisocyanate with (CH.sub.3).sub.3.Si--O--Na dissolved in the aqueous phase, the reactions being accompanied by film formation. These droplets pre-encapsulated by the formation of silicone polymers are then "encapsulated" by coacervation (for example with oppositely charged polymers in accordance with U.S. Pat. No. 2,800,457).
German Offenlegungsschrift No. 15 70 548 describes a one-component system of prolonged stability which consists of a mixture of 1 mole of a polyester, polyether or polythioether, at least 1.5 moles of a solid isocyanate containing uret dione groups and having a melting point of .gtoreq.100.degree. C. and at least 0.3 mole of a solid chain-extending agent containing OH-- and/or NH.sub.2 -- groups and having a melting point of .gtoreq.80.degree. C. In this known system, at least 80% of the solid constituents of the mixture have to have a particle size of .ltoreq.30 .mu.m. The stability of the product in storage at room temperature amounts to between a few days and a few weeks and, at 50.degree. C., to only a few hours. One of the disadvantages of the process lies in the fact that, of three reactants, at least two must be present in solid form to guarantee the requisite stability in storage. As a result, the viscosity of the mixture obtained is generally very high and continues to increase slowly because none of the compounds is modified in its reactivity.
The reaction on the surface of the solid particles, which is reflected in the continuous increase in viscosity, takes place without control and too slowly for practical purposes and does not retard the reactivity of the polyisocyanates sufficiently for the system to become self-stabilizing. In addition, the high percentages of solid constituents can be expected to give rise to inhomogeneities in the heated product during hardening of the mixture. In addition, difficulties are involved in processing the highly viscous or solid mixtures because, in contrast to liquid mixtures, they have first to be brought into a formable state by an increase in temperature or by the application of pressure. The residence of high-melting polyisocyanates in mixtures of high and low molecular weight polyols is accompanied by a continuous and relatively fast further reaction producing a considerable increase in viscosity. In other words, the surface reaction on the solid polyisocyanate particles does not form a coating around the polyisocyanate which is sufficient to retard its reactivity, i.e. which has an adequate stabilizing effect.
British Patent No. 1,134,285 describes a process for the production of dimeric diisocyanates in an aqueous reaction medium. According to this patent, the dimers thus produced in aqueous suspension do not react with polyfunctional compounds containing active hydrogen atoms at room temperature, but instead may be thermally crosslinked to form polyurethanes. Stability may possibly be brought about by a slow surface reaction of isocyanate groups with water. Crosslinking is subsequently obtained by splitting of the uret dione ring at elevated temperatures, for example at temperatures in the range from 150.degree. to 200.degree. C.